Resistance heater means are already known as the starting aid in high pressure sodium vapor lamps. For example, in U.S. Pat. No. 4,001,634, there is described thermal starter means for this type lamp in the form of a bimetal switch which is located outside the conventional ceramic arc tube and which upon opening responsive to the applied electric current generates an inductive voltage pulse said to be sufficient to ionize the starting gas in the arc tube and start lamp operation. It is also known that the color temperature of high pressure sodium vapor lamp operation varies undesirably with the overall lamp color point depending to a significant degree on the temperature of the cold spot in the arc tube where amalgam condensation takes place. The "cold spot" temperature controls amalgam vapor pressure inside the arc tube thereby increasing or decreasing the amount of vaporized amalgam which produces the visible lamp emission as well as the color of said lamp emission.
Resistance heater means have also been located inside the arc tube of the sodium vapor lamp to stabilize the cold spot temperature and amalgam vapor pressure which tend to rise during lamp operating life. The undesired variation is attributable to operating voltage rise that eventually reaches the ballast sustaining voltage and determines the end of lamp life in this type lamp. Said resistance heater means is supplied with the lamp current which inherently decreases with an increase in the lamp operating voltage and reduces supplemental heating of the amalgam reservoir as the stabilizing mechanism for improved lamp operation. A representative sodium vapor lamp incorporating this type resistance heater means is described in U.S. Pat. No. 3,851,207, wherein the heater element simply comprises an integral part of the thermionic electrode. Understandly, the power consumed by such heater means reduces the lamp operating efficiency and this power loss will be greatest at the initial lamp operation. Since the present lamp designs exhibit only a 20 volt rise after 15,000 hours of lamp operation, such loss in lamp operating efficiency seems excessive especially when encountered before any voltage rise takes place.
Accordingly, it would be desirable to provide improved supplemental heating means whereby the vaporized amalgam in this type lamp can be adjusted to compensate for the inherent operating voltage rise. It would also be desirable to do so in a manner which does not add significant cost to the lamp design or its cost of manufacture by requiring a modification in the lamp ballast circuitry.